Layer in the rate/production capacity capability

[RobertLRead]

At present Supply objects and SupplyTrees are agnostic as to time---they represent supply relationships, but do not capture the rate or capacity to deliver goods. This is a fairly deep addition to our current mathematical structure, but it would allows us to model orders much more accurately.

It will remain the case that one might want to have discrete orders and continuous orders. I recommend we start with continuous orders first as something that is mathematically simpler.